The present invention relates to pressure transducers. More specifically, the present invention relates to pressure transducers with compensation for thermal transients.
FIG. 1 shows a sectional view of a prior art unheated capacitive pressure transducer 100. Transducer 100 includes several major components such as an external shell 110, a capacitive pressure sensor 140, a front end electronics assembly 160, and an input/output (I/O) electronics assembly 180. In operation, transducer 100 generates an output signal indicative of a pressure measured by sensor 140.
For convenience of illustration, many mechanical details of transducer 100, such as the construction of sensor 140 and the mounting of sensor 140 and electronics assemblies 160, 180, have been omitted from FIG. 1. However, capacitive pressure transducers such as transducer 100 are well known and are described for example in U.S. Pat. Nos. 5,625,152 (Pandorf); 5,911,162 (Denner); and 6,029,525 (Grudzien).
Briefly, external shell 110 includes a lower enclosure 112, an upper electronics enclosure 114, and a joiner 116 that holds enclosures 112, 114 together. Sensor 140 and front end electronics assembly 160 are disposed in the space defined by lower enclosure 112 and joiner 116. Mounting posts 162 support front end electronics assembly 160 over sensor 140 and wires 164 electrically connect front end electronics assembly 160 and sensor 140. The I/O electronics assembly 180 is disposed in the space defined by upper electronics enclosure 114 and joiner 116 and is electrically connected to front end electronics assembly 160.
Sensor 140 includes a metallic, flexible, diaphragm 142 and a pressure, or inlet, tube 144. Tube 144 extends from an area proximal to the diaphragm through the lower sensor enclosure 112. The lower, or external, end of tube 144 is generally coupled to a source of fluid (not shown). Pressure of fluid in the source is communicated via tube 144 to the lower surface of diaphragm 142 and the diaphragm 142 flexes up or down in response to changes in pressure within tube 144. Diaphragm 142 and a reference conductive plate of sensor 140 form a capacitor, and the capacitance of that capacitor varies in accordance with movement or flexion of the diaphragm. Accordingly, that capacitance is indicative of the pressure within tube 144. Front end electronics assembly 160 and I/O electronics assembly 180 cooperatively generate an output signal representative of the capacitance of sensor 140 which is, of course, also representative of the pressure within tube 144. I/O electronics assembly 180 makes that output signal available to the environment external to transducer 100 via an electronic connector 182.
FIG. 2 shows one example of how a capacitive pressure sensor 140 can be constructed. Capacitive pressure sensors of the type shown in FIG. 2 are discussed in greater detail in U.S. Pat. No. 6,029,525 (Grudzien). The sensor 140 shown in FIG. 2 includes a circular, conductive, metallic, flexible diaphragm 142, a pressure tube 144, and an electrode 246. Electrode 246 and diaphragm 142 are mounted within a housing 248. Electrode 246 includes a ceramic block 250 and a conductive plate 252. The ceramic block 250 is rigidly mounted to the housing 248 so that a bottom face of block 250 is generally parallel to, and spaced apart from, the diaphragm. The bottom face of block 250 is normally planar and circular. The conductive plate 252 is deposited onto the bottom face of block 250 and is also generally parallel to, and spaced apart from, the diaphragm. Conductive plate 252 and diaphragm 142 form two plates of a variable capacitor 254. The capacitance of capacitor 254 is determined in part by the gap, or spacing, between the diaphragm 142 and the conductive plate 252. Since the diaphragm flexes up and down (thereby changing the spacing between diaphragm 142 and conductive plate 252) in response to pressure changes in tube 144, the capacitance of capacitor 254 is indicative of the pressure within tube 144.
FIG. 2 shows only one of the many known ways of configuring a capacitive pressure sensor 140. However, capacitive pressure sensors 140 generally include one or more conductors that are held in spaced relation to a flexible, conductive, diaphragm. The diaphragm and the conductors form plates of one or more variable capacitors and the capacitance of those capacitors varies according to a function of the pressure in tube 144.
Capacitive pressure sensors often include additional features such as a tube 260 and a getter 262 as shown in FIG. 2. When sensor 140 is being constructed, tube 260 is initially open and is used to establish a reference pressure (e.g., vacuum) in the portion of housing 248 above diaphragm 142. Once the desired reference pressure is established (e.g., by attaching a vacuum pump to tube 260), the upper portion of tube 260 is closed, or xe2x80x9cpinched offxe2x80x9d, so as to maintain the desired reference pressure inside the upper portion of housing 248. Getter 262 is often included to absorb gas molecules that get into the upper portion of housing 248 after tube 260 has been pinched off (e.g., via outgasing of electrode 250).
Returning to FIG. 1, in operation, front end electronics assembly 160 measures the capacitance (or capacitances) of sensor 140 and generates a voltage signal representative of the pressure in inlet tube 144 (e.g., the voltage signal may vary linearly with changes in pressure). I/O electronics assembly 180 typically amplifies and further conditions that voltage signal to generate the output signal of transducer 100.
As shown, the distance between I/O electronics assembly 180 and sensor 140 is greater than the distance between front end electronics assembly 160 and sensor 140. The front end electronics assembly 160 is mounted close to sensor 140 so as to reduce the length of the wires 164. Reducing the length of wires 164 reduces the capacitance of those wires and in effect reduces a stray capacitance associated with the capacitance of sensor 140 and thereby improves the accuracy of transducer 100. I/O electronics assembly 180 is normally separated from sensor 140 by a greater distance so that heat generated by assembly 180 does not adversely affect operation of sensor 140.
Ideally, the output signal of transducer 100 varies only in response to changes in the pressure of the fluid in tube 144. However, changes in temperature of the ambient environment can also affect the output signal. This is primarily due to the different coefficients of thermal expansion of different materials used to construct the sensor 140. Many transducers incorporate heaters and heater shells to provide a controlled thermal environment for the pressure sensor and thereby minimize the affect of changes in the ambient temperature. However, unheated transducers such as transducer 100 also often provide mechanisms for compensating for temperature changes in the ambient environment.
Transducer 100 includes a temperature sensor S1, which is mounted on the I/O electronics assembly 180. In operation, temperature sensor S1 generates an output signal TS1, which is representative of the temperature of sensor S1. The sensor S1 is positioned so that its temperature (and its output signal TS1) is representative of the temperature of the ambient environment of transducer 100. In operation, transducer 100 uses the temperature sensor S1 to provide compensation for ambient temperature changes. To provide this temperature compensation, I/O electronics assembly 180 generates the output signal of the transducer 100 according to the following Equation (1).
OT=OU+ƒ1(TS1)xe2x80x83xe2x80x83(1)
In Equation (1), OT is the output signal of transducer 100 (a voltage representative of the pressure of the fluid in tube 144); OU is an internal signal generated within transducer 100 that is representative of the pressure in tube 144; and f1(TS1) is a function of the ambient temperature as measured by temperature sensor S1. OU may be called the xe2x80x9cuncompensated output signalxe2x80x9d because it has not yet been compensated for temperature changes. OU may be the output signal generated by front end electronics assembly 160, or alternatively it may be an internal signal generated by I/O electronics assembly 180 (e.g., it may represent the output signal generated by front end electronics assembly 160 after amplification or linearization by I/O electronics 180 but prior to application of some other form of compensation). The function f1(TS1) is given by the following Equation (2).
ƒ1(TS1)=C1(TS1xe2x88x92TS1Reƒ)xe2x80x83xe2x80x83(2)
In Equation (2), TS1 is the output signal generated by temperature sensor S1, and the terms C1 and TS1Ref are constant value parameters, the values of which are determined by a calibration procedure. In the calibration procedure, a reference pressure of zero, or a vacuum, is applied to the inlet tube 144. Ideally, the uncompensated output signal OU is zero volts whenever a vacuum is applied to inlet tube 144. However, due to the temperature sensitive performance of pressure sensor 140, the uncompensated output signal OU normally varies with changes in ambient temperature. TS1Ref is selected by first determining the ambient temperature at which the uncompensated output signal OU actually does equal zero volts when a vacuum is applied to the pressure inlet tube, and by then setting TS1Ref equal to the value of the output signal TS1 generated by sensor S1 at that temperature.
FIG. 3 illustrates the procedure used for determining the value of the constant C1. As shown in FIG. 3, transducer 100 is operated at a first ambient temperature TA1 until a time T2. Between times T2 and T3 heat is added to the ambient environment of transducer 100 until the ambient temperature has risen to a new value TA2. At times T1 and T4 measurements are made of the uncompensated output signal OU while a vacuum reference pressure is applied to the inlet tube 144. T1 is selected to be a time after which transducer 100 has been operating sufficiently long at the first ambient temperature TA1 such that the transducer 100 has reached a steady state. Similarly, time T4 is selected to be a time after which transducer 100 has been operating sufficiently long at the second ambient temperature TA2 such that the transducer 100 has reached a steady state at the new temperature TA2. Using the measurements of the uncompensated output signal OU made at times T1 and T4, the value of the constant C1 is calculated according to the following Equation (3).                               C          1                =                              [                                                            O                  U                                ⁡                                  (                                      T                    4                                    )                                            -                                                O                  U                                ⁡                                  (                                      T                    1                                    )                                                      ]                                (                                          T                A2                            -                              T                A1                                      )                                              (        3        )            
In Equation (3), the term OU(T1) is the value of the uncompensated output signal OU at time T1 (or the steady state value of the uncompensated output signal OU at the first ambient temperature TA1). Similarly, the term OU(T4) is the value of the uncompensated output signal OU at time T4 (or the steady state value of the signal OU at the second ambient temperature TA2).
The values of the parameters C1 and TS1Ref measured by the above described calibration procedure are typically unique for every transducer and normally depend on minor variations or tolerances in the manufacture of each transducer. So, for example, two seemingly identical units of transducer 100 may have very different values for the parameters C1 and TS1Ref. Accordingly, measurement of these parameters for each transducer is part of the normal manufacturing process for transducer 100.
I/O electronics assembly 180 normally includes read only memory (ROM) for digitally storing the values of the parameters C1 and TS1Ref within transducer 100. I/O electronics assembly 180 also normally includes a digital processor for computing Equation (1) and generating a digital output representative of the transducer""s output signal OT. I/O electronics assembly 180 may further include a digital-to-analog converter for generating an analog output signal OT.
Although the above described method compensates the transducer""s output signal to some extent for changes in ambient temperature, it would still be advantageous to develop improved methods and structures for providing improved temperature compensation.
These and other objects are provided by an improved method of providing temperature compensation in a pressure transducer. Prior art pressure transducers provide compensation for steady state temperature changes in the ambient environment. In addition to providing steady state compensation, pressure transducers constructed according to the invention may also provide compensation for transient temperature changes in the ambient environment. As is known, pressure transducers may provide many forms of compensation (e.g., amplification, linearization over a desired range, steady state temperature compensation). In one aspect, transient temperature compensation of the invention is one additional form of compensation provided by the transducer. The transient temperature compensation may be provided by adding a transient compensation term to an internal signal generated by the transducer, where the internal signal can be, for example, a signal representative of a measured pressure prior to application of any other forms of compensation have been provided, or after one or more forms of other compensation have been provided. The transient compensation term may be generated in a variety of ways and is preferably a function of a difference between a temperature of the pressure sensor and a temperature of the ambient environment.